This research is directed toward elucidation of the metabolism of molybdenum and the molybdenum-cofactor in biological systems. The significance of this research centers on the fact that Mo is an essential trace element for virtually all bacteria, plants, and animals by virtue of its function in nitrate reductase, nitrogenase, formate dehydrogenase, sulfite oxidase, xanthine oxidase and aldehyde oxidase. In all of these enzymes (except nitrogenase) Mo is incorporated into a cofactor that is a novel pterin. In contrast to the function of folic acid and biopterin, which are other pterins of biological significance, the pterin of Mo-cofactor is a tightly-bound prosthetic group of molybdoenzymes and represents a heretofore unknown function for pterins in biological systems. The biosynthesis of Mo-cofactor will be elucidated in E. coli initially. Cells will be grown in media with 14C, 35S, or 99Mo-labeled precursors, and labeled cofactor and its biosynthetic intermediates will be detected by immunoelectrophoresis and HPLC. Studies will be performed to discover if Mo-cafactor shares any biosynthetic intermediates with folic acid and biopterin. Next, mammalian utilization of dietary sources of Mo-cofactor will be studied by examining the metabolic fate of labeled Mo-cofactor fed to rats. The rat molybdoenzymes will be isolated by antibody precipitation and the Mo-cofactor released to determine if dietary sources of labeled Mo-cofactor were specifically incorporated. Studies on the regulation of Mo-cofactor will determine if an enzymatic process is essential for incorporation of Mo into the cofactor and if Mo must induce this enzyme. Also the role of the Mo-cofactor carrier in the assembly of nitrate reductase subunits into a functional, membrane-associated protein will be investigated. This research will further the understanding of the human genetic deficiences of sulfite oxidase, xanthine oxidase, and Mo-cofactor, as well as provide the first information on the role of dietary Mo in the regulation of pterin metabolism.